in theory -rc1 is supposed to have all the major new stuff in it, but in practice Linus
frequently uses -rc1 as a mid-merge status point, getting large numbers of additional changes
shortly after -rc1 (mostly from andrew and other top maintainers) In some cases this is
becouse the changes introduced in -rc1 require adapting other patches to work with them
(something that -next may help with), in other cases it's that people like andrew are busy
checking and reviewing patches during the -rc1 timeframe, and so Linus seems to pull from them
just after it.
so I suspect that your table of changes per -rc would show an even bigger drop-off after -rc2
then it does after -rc1.